The notation 1/2 + 1/4 + 1/8 + 1/16 + ... is, loosely speaking, alternative notation for the limit as n approaches infinity of the finite sum of the series 1/2 + 1/4 + ... + 1/2^n. This finite sum is of course 1 - 1/2^n. Hence, 1/2 + 1/4 + 1/8 + ... = lim n->inf (1 - 1/2^n) = 1. Doronshadmi's assertion that 1/2 + 1/4 + 1/8 + ... < 1 is complete nonsense for that reason. For any natural number n, it is true that 1/2 + 1/4 + 1/8 + ... + 1/2^n < 1. But, having a "+ ..." at the end indicates that the limit is being taken.
No. The notation
1/2 + 1/4 + 1/8 + 1/16 + ... is a descriptive and non-algebraic expression that merely establishes an infinite series. Since Doron is not familiar with the established symbolic language, he would use the description to indicate inequality
1/2 + 1/4 + 1/8 + 1/16 + ... < 1. This symbolism is normally avoided, coz it lacks necessary elements to prove or disprove the assertion. When this convergent series is converted into its algebraic form including the inequality
1 - 1/2
n < 1 where n → ∞
it can be shown in three easy steps that the inequality holds.
1 - 1/2
n < 1
-1/2
n < 0
-1 < 0
However, the solution of the inequality is also good for any sum
S ≥ 1, which allows the sum be greater than 1. That's why the limit needs to be computed. During the process
[
lim n → ∞] 1 - 1/2
n
the whole term
1/2n is held sufficiently close to zero to be removed under the rule that derives the limits. Obviously, the term never equals zero as
n goes to infinity, and therefore the series cannot reach its limit.
No. The partial sums of the series are always less than 1 for any given natural number n. The infinite sum is defined as the limit of the sequence of partial sums, and the value of that limit is 1 in this particular case.
The conversion of the series into function
f(x) = 1 - 1/2x where
x → ∞ leaves the issue of partial sums irrevelant, coz
f(x) is a function and not a series comprising addends. But the "partiality" can be inherited and resurface in an assertion that cannot be easily defined. Let
f(x) = 1 - 1/2
x where x → ∞
and
g(x) = 1 - 1/2
x where x ≥ 1
In that case,
f(x) ≠ g(x) as it is often asserted.
The concept of "partiality" must be properly translated into the function that substitutes the series and that's the point where it squeaks.